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Topic: Why works 5110 LCD without GND pin (Read 1 time) previous topic - next topic



today I connected this LCD and it works properly.

But what I cant understand is why LCD works even if I disconnect GND pin. If I disconnected it, everything is still workind including backlight.

Second question is about backlight control. I drive it by PWM output (256 is power off bcklght, 0 is full power). Why bcklght work if i Connect it to GND (i think it isnt same as 0 on PWM pin) and why it doesnt work, if it is not connected?


Mar 06, 2014, 02:24 am Last Edit: Mar 06, 2014, 02:28 am by Nick_Pyner Reason: 1
Aha! very confusing.

I think you have the same LCD as me, they are wired differently to most others, and I got mine going more or less by accident.

Code: [Select]
7?LIGHT--------- backlight control terminal

Is just a rather pompous way of saying LED to ground. Just do it and it will  be fine.

My displays are on shields, so I can't test anything. If it works without ground pin 8 connected, I guess it is getting a ground through pin 7 LED. I guess it will stop if you disconnect pin 7 too

As for the second question, it is garbled, but perhaps the above answers it. It's not clear if your PWM control works at all, I assume it doesn't. If that is the case, I wouldn't bother anyway. This particularly applies if the backlights are blue. They are so useless you are likely to find they are better left disconnected.


I do believe I have one or two of those but only got so far as to check that the backlight worked and then put them aside.

It is most likely that if the ground is not connected, whichever of the data pins at any moment is pulled low, supplies a ground for the device through "parasitic diodes" on the chip and any capacitor on the board will hold the voltage as the data flows.  It does not really make sense however to operate it in such a fashion - just ensure the ground is connected..


That might be a better explanation. If the 5110 keeps going when pin7 is disconnected, it will surely prove it.


Parasitic powering through the protection diodes like this is likely to damage the
chip - avoid doing this if you possibly can.  All CMOS logic chips work like this
since the inputs have to be protected from static electricity which would otherwise
destroy the input FETs as soon as you handled the chip (even capacitive mains pickup
by the human body is plenty to blast away gate oxide on a logic MOSFET, if only takes
10V or so for a 5V chip.)

The protection diodes typically can take 1mA or similar continuous, but put more through
them and they will overheat, degrade, perhaps fail, perhaps put the whole chip into
latch-up and cook it if the current is really large.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Here are pics of connections(with and without PWM controlled backlight). So, is it that if data pins is on low leve, it is serve as GND (in very simplified way)?


So, is it that if data pins is on low leve, it is serve as GND (in very simplified way)?

Yes. That is what Paul was saying back in reply #2.

And, as Mark said in reply #4, it is not a good idea to do that.


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